1. Field of the Invention
The present invention relates to an imaging method. More particularly, it relates to an imaging method for performing image acquisition while displacing and without halting the field-of-view in an observation apparatus such as scanning electron microscope.
2. Description of the Related Art
A review apparatus has been known as an apparatus for observing in detail the foreign substances or defects on a semiconductor wafer. More concretely, this is the following apparatus: First, positions of the foreign substances or defects on the semiconductor wafer are identified in advance using some other apparatus (e.g., SEM or optical external-appearance inspection apparatus). Then, this wafer is displaced, and the defect locations on the wafer are observed based on the measured position information, thereby observing (i.e., reviewing) the detailed images. This review apparatus is mainly used for investigating the cause for occurrence of the foreign substances or defects, and for enhancing the yield in semiconductor-chip fabrication process. In the apparatus like this, it is required to inspect enormous number of wafers and enormous number of foreign substances or defects in a short time and with a high efficiency. This requirement further requires that the review apparatus itself eliminate wastes on the observation down to the lowest possible degree, and thereby execute the high-efficiency review. Hereinafter, in the present specification, the foreign substances or defects in general will be referred to as just “defects”.
In the review apparatus, a stage on which the wafer is mounted is displaced up to the field-of-view center of the observation apparatus in an observation system, such as optical microscope or electron microscope. Next, the content that the observation system has photographed is stored and acquired into memory or storage device in a computer. Moreover, the images acquired are subjected to image processing, thus identifying accurate positions of the defects, and magnifying the accurate positions. Otherwise, the magnified images are acquired, thereby performing the detection and photographing of the defects. In order to detect a defect, in some cases, the stage is displaced to the position of the in-chip coordinate which is the same as that of the defect within a chip that is one-chip adjacent to the chip where the defect exists. Then, in this case, the review apparatus acquires an image for the comparison which is referred to as “reference image”. At this time, usually, the stage is displaced to the image acquisition position, and there, the stage is halted, and then the image acquisition is performed. In the image acquired by the electron microscope or the like, which is accompanied by a comparatively bad S/N ratio, it is required to implement an enhancement in the S/N ratio by integrating plural pieces of images concerned therewith. If the image is acquired without halting the stage, the resultant image turns out to become a one which is blurred in the displacement direction. This situation, conventionally, has required that the review apparatus halt the stage at the image acquisition positions, and then acquire the plural pieces of images.
As a method of acquiring an image while displacing the stage, there has been known a one disclosed in JP-A-2002-310962. This method is as follows: First, displacement velocity of the stage is determined in advance. Then, the location to be scanned is changed in response to displacement quantity of the stage, so that the scan area will not displace even if the stage has been displaced.
In the method where the location to be scanned is changed in response to the displacement quantity of the stage, there are some cases where position coordinate of the stage and coordinate of the location at which the observation is actually performed do not necessarily coincide with each other. This phenomenon results from errors caused by physical factors of the stage, such as thermal deformation of the stage, height error of the measurement position, and error due to rigidity of the stage, and also the other factors such as the displacement and rotation of the wafer during the displacement of the stage. These errors attain to as much as 1 μm in some cases. Meanwhile, in recent years, significant development has been made in miniaturization of the defects to be detected. As a result, the observation at a high magnification is now becoming more and more required. For example, observing a defect of 45 nm or less requires that the field-of-view range of order of 1 μm be observed with a high definition. In this way, the higher the magnification becomes, the more incapable it becomes to neglect the errors as mention above. This situation makes it more difficult to irradiate one and the same location with the beam in accompaniment with the displacement of the field-of-view.